1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. These are physically and electrically configured in such a manner that they provide cooling or heating. Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in H. J. Goldsmid, Electronic Refrigeration, Pion Ltd., 207 Brondesbury Park, London, NW2 5JN, England (1986). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1. Generally, P-type and N-type thermoelectric elements 102 are arrayed in a rectangular assembly 100 between two substrates 104. A current, I, passes through both element types. The elements are connected in series via copper shunts 106 soldered to the ends of the elements 102. A DC voltage 108, when applied, creates a temperature gradient across the TE elements. TE""s are commonly used to cool liquids, gases and objects. FIG. 2 for flow and FIG. 3 for an article illustrate general diagrams of systems using the TE assembly 100 of FIG. 1.
The basic equations for TE devices in the most common form are as follows:
qc=xcex1ITcxe2x88x92xc2xdI2Rxe2x88x92Kxcex94Txe2x80x83xe2x80x83(1)
xe2x80x83qin=xcex1Ixcex94T+I2Rxe2x80x83xe2x80x83(2)
qh=xcex1ITh+xc2xdI2Rxe2x88x92Kxcex94Txe2x80x83xe2x80x83(3)
where qc is the cooling rate (heat content removal rate from the cold side), qin is the power input to the system, and qh is the heat output of the system, wherein:
xcex1=a Seebeck Coefficient
I=Current Flow
Tc=Cold side absolute temperature
Th=Hot side absolute temperature
R=Electrical resistance
K=Thermal conductance
Herein xcex1, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges.
Under steady state conditions the energy in and out balances:
qc+qin=qhxe2x80x83xe2x80x83(4)
Further, to analyze performance in the terms used within the refrigeration and heating industries, the following definitions are needed:                     β        =                                            q              c                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              =                      Cooling  Coefficient  of  Performance  (COP)                                              (        5        )                                γ        =                                            q              h                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              =                      Heating  COP                                              (        6        )            
From (4);                                                         q              c                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                              +                                    q              in                                      q                              i                ⁢                                  xe2x80x83                                ⁢                n                                                    =                              q            h                                q                          i              ⁢                              xe2x80x83                            ⁢              n                                                          (        7        )                                          β          +          1                =        γ                            (        8        )            
So xcex2 and xcex3 are closely connected, and xcex3 is always greater than xcex2 by unity.
If these equations are manipulated appropriately, conditions can be found under which either xcex2 or xcex3 are maximum or qc or qh are maximum.
If xcex2 maximum is designated by xcex2m, and the COP for qc maximum by xcex2c, the results are as follows:                               β          m                =                                            T              c                                      Δ              ⁢                              xe2x80x83                            ⁢                              T                c                                              ⁢                      xe2x80x83                    ⁢                      (                                                                                1                    +                                          ZT                      m                                                                      -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          ZT                      m                                                                      +                1                                      )                                              (        9        )                                          β          c                =                  (                                                                      1                  2                                ⁢                                  xe2x80x83                                ⁢                                  ZT                  c                  2                                            -                              Δ                ⁢                                  xe2x80x83                                ⁢                T                                                                    ZT                c                            ⁢                              T                h                                              )                                    (        10        )            
where;                     Z        =                                            α              2                        RK                    =                                                                      α                  2                                ⁢                ρ                            λ                        =                          Figure  of  Merit                                                          (        11        )                                          T          m                =                                            T              c                        +                          T              h                                2                                    (        12        )            xe2x80x83R=xcfx81xc3x97length/areaxe2x80x83xe2x80x83(13)
K=xcexxc3x97area/lengthxe2x80x83xe2x80x83(14)
xcexxc3x97Material Thermal Conductivity;xe2x80x83xe2x80x83(15)
and
xcfx81=Material Electrical Resistivityxe2x80x83xe2x80x83(16)
xcex2m and xcex2c depend only on Z Tc and Th. Thus, Z is named the figure of merit and is basic parameter that characterizes the performance of TE systems. The magnitude of Z governs thermoelectric performance in the geometry of FIG. 1, and in most all other geometries and usages of thermoelectrics today.
For today""s materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are too low to compete with those of most refrigeration systems employing freon-like fluids (such as those used in refrigerators, car HVAC systems, building HVAC systems, home air conditioners and the like).
The limitation becomes apparent when the maximum thermoelectric efficiency from Equation 9 is compared with Cm, the Carnot cycle efficiency (the theoretical maximum system efficiency for any cooling system);                                           β            m                                C            m                          =                                                                              T                  c                                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  T                                            ⁢                              xe2x80x83                            ⁢                              (                                                                                                    1                        +                                                  ZT                          m                                                                                      -                                                                  T                        h                                                                    T                        c                                                                                                                                                1                        +                                                  ZT                          m                                                                                      +                    1                                                  )                                                                    T                c                                            Δ                ⁢                                  xe2x80x83                                ⁢                T                                              =                      (                                                                                1                    +                                          ZT                      m                                                                      -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          ZT                      m                                                                      +                1                                      )                                              (        17        )            
Note, as a check if Zxe2x86x92∞,xcex2xe2x86x92Cm.
Several commercial materials have a ZTA approaching 1 over some narrow temperature range, but ZTA is limited to unity in present commercial materials. Typical values of Z as a function of temperature are illustrated in FIG. 4. Some experimental materials exhibit ZTA=2 to 4, but these are not in production. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
Several configurations for thermoelectric devices are in current use in applications where benefits from other qualities of TEs outweigh their low efficiency. Examples of uses are in automobile seat cooling systems, portable coolers and refrigerators, liquid cooler/heater systems for scientific applications, the cooling of electronics and fiber optic systems and for cooling of infrared sensing system.
All of these commercial devices have in common that the heat transport within the device is completely constrained by the material properties of the TE elements. In sum, in conventional devices, conditions can be represented by the diagram in FIG. 5. FIG. 5 depicts a thermoelectric heat exchanger 500 containing a thermoelectric device 501 sandwiched between a cold side heat exchanger 502 at temperature TC and a hot side heat exchanger 503 at temperature TH. Fluid, 504 at ambient temperature TA passes through the heat exchangers 502 and 503. The heat exchangers 502 and 503 are in good thermal contact with the cold side 505 and hot side 506 of the TE 501 respectively. When a DC current from a power source (not shown) of the proper polarity is applied to the TE device 501 and fluid 504 is pumped from right to left through the heat exchangers, the fluid 504 is cooled to TC and heated to TH. The exiting fluids 507 and 508 are assumed to be at TC and TH respectively as are the heat exchangers 502 and 503 and the TE device""s surfaces 505 and 506. The temperature difference across the TE is xcex94T.
None of the existing TE assemblies modify the thermal power transport within the TE assembly by the application of outside influences. An improved efficiency thermoelectric device is achieved by generally steady state convective heat transport within the device itself. Overall efficiency may be improved by designing systems wherein the TE elements are permeable to the flow of a heat transport fluid, transport thermal energy to a moving substance, or move the TE material itself to transport thermal energy. It should be noted that the term xe2x80x9cheat transportxe2x80x9d is used throughout this specification. However, heat transport encompasses thermal energy transfer of both removing heat or adding heat, depending on the application of cooling or heating.
One aspect of the present invention involves a thermoelectric system having a plurality of thermoelectric elements forming a thermoelectric array. The array has at least one first side and at least one second side exhibiting a temperature gradient between them during operation. In accordance with the present invention, at least a portion of the thermoelectric array is configured to facilitate convective heat transfer through the array. To accomplish this, the array is configured to permit flow of at least one convective medium through the at least a portion of the array to provide generally steady-state convective heat transport toward at least one side of at least a portion the thermoelectric array. The thermoelectric system may be used for cooling, heating or both cooling and heating.
In one embodiment, the convective medium flows through at least some of the thermoelectric elements or along the length, between and/or around the thermoelectric elements. In another embodiment, the convective medium flows both along and through the thermoelectric elements. In one preferred embodiment, to permit flow through the thermoelectric elements, the elements may be permeable or hollow. A combination of both permeable and hollow elements may also be used in an array. In one embodiment, the elements are porous to provide the permeability. In another embodiment, the elements are tubular or have a honeycomb structure.
In one embodiment, flow of the convective medium occurs in a single general direction, such as from the first side to the second side or from a point between the first and second sides toward the first side or the second side. In another embodiment, the convective medium flows in at least two general directions, such as from between the first side and the second side toward the first side and toward the second side. All such flows may be generally within or along the length of the thermoelectric elements (including in a spiral) or a combination thereof.
In one particular embodiment, at least some of the thermoelectric elements form concentric tubes with convective medium flow between the concentric tubes. In one embodiment, a first set of concentric tubes forms a thermoelectric element, with each tubular portion made from thermoelectric material of the same conductivity type as the next tubular portion in the set of concentric tubes. In such an embodiment, a second set of concentric tubes is formed of a thermoelectric material of a different conductivity type from the first set. Alternatively, the tubes may concentrically alternate between p-type thermoelectric material and n-type thermoelectric material.
In another embodiment, at least part of the convective medium is thermoelectric material. The convective medium thermoelectric material forms at least some of the thermoelectric elements. In another embodiment, at least part of the convective medium is thermoelectric material, with the convective medium thermoelectric material forming a first portion of at least some of the thermoelectric elements, and a solid thermoelectric material forming a second portion of the same thermoelectric elements. For example, the solid thermoelectric material is tubular or otherwise hollow, and the convective medium thermoelectric material flows through the solid thermoelectric material. The combination forms at least some thermoelectric elements. In one embodiment, the convective medium is a fluid, such as air, a solid or a combination of a fluid and a solid such as a slurry.
In one configuration, a first plurality of the thermoelectric elements are configured for convective heat transport of a first type and a second plurality of the thermoelectric elements are configured for convective heat transport of a second type.
For example, the first plurality of thermoelectric elements may be permeable, and the second plurality may be thermoelectric elements made from the convective material moving through the array. An example of a division of elements is the first plurality being thermoelectric elements of a first conductivity type and the second plurality being thermoelectric elements of a second conductivity type. In another embodiment, at least some of the thermoelectric elements do not utilize convection, while others are configured for convection. For example, the thermoelectric elements that do not utilize convection are of a first conductivity type and the thermoelectric elements that utilize convection are of a second conductivity type.
Preferably, at least a portion of the array has at least one heat transfer feature that improves heat transfer between at least some of the convective medium and at least some of the thermoelectric elements. For example, where the thermoelectric elements are tubular or otherwise hollow, the heat transfer feature is inside at least some of the thermoelectric elements. Where the convective medium flows along the outside of the thermoelectric elements, the heat transfer feature is between at least some of the thermoelectric elements. An example of such heat transfer feature is a convective medium flow disturbing feature.
Another aspect of the present invention involves a method of improving efficiency in a thermoelectric system having a plurality of thermoelectric elements forming a thermoelectric array. The thermoelectric array has at least one first side and at least one second side exhibiting a temperature gradient between them during operation of the thermoelectric array. The method involves actively convecting thermal power through at least a portion of the array in a generally steady-state manner. Generally, the step of convecting thermal power involves flowing at least one convective medium through at least a portion of the thermoelectric array. The convective medium may be fluid, solid or a combination of fluid and solid. The method may be used for cooling, for heating or for both cooling and heating applications.
In one advantageous embodiment, the step of flowing involves flowing at least some of the convective medium through at least some of the thermoelectric elements. For example, the thermoelectric elements are constructed to be permeable or porous. The thermoelectric elements may also be hollow, such as having a tubular or honeycomb configuration.
In one embodiment, the step of flowing involves flowing the convective medium generally through the array from the first side to the second side, or generally from between the first side and the second side toward the first side or toward the second side. In another embodiment, the step of flowing involves flowing the convective medium in at least two general directions, such as flowing the convective medium generally from between the first side and the second side toward the first side and toward the second side. The flow may be through at least some of the thermoelectric elements, along at least some of the thermoelectric elements, through some thermoelectric elements and along others, or any combination.
In one embodiment, the thermoelectric material forms at least a portion of the convective medium. In this embodiment, the method further involves the step of forming a first portion of at least some of the thermoelectric elements with the convective material. As a further alternative, the method in this configuration further involves the step of flowing the convective medium thermoelectric material through other thermoelectric material in a hollow form, the combination of the flowing convective medium thermoelectric material and the thermoelectric material in a hollow form forming the at least some thermoelectric elements.
In one embodiment of the method, the step of actively convecting involves convecting heat through a first portion of the array in a first manner and through a second portion of the array in a second manner. For example, the first portion of the array is a plurality of thermoelectric elements of a first conductivity type and the second portion of the array is a plurality are thermoelectric elements of a second conductivity type.
Yet another aspect of the present invention involves a thermoelectric system with a thermoelectric array having a plurality of thermoelectric elements and having at least one first side and at least one second side. The first and second sides exhibit a temperature gradient between them during operation. At least a portion of the thermoelectric array is configured to permit flow of at least one convective medium through the at least a portion of the array to provide generally steady-state convective heat transport toward at least one side of at least a portion the thermoelectric array.
According to this aspect of the present invention, the system has at least one control system, with at least one controller, at least one input coupled to the controller, and at least one output coupled to the controller and to the thermoelectric array. The output is advantageously controllable by the controller to modify at least one characteristic of at least a portion of the thermoelectric array. The at least one input may be at least one external sensor, at least one sensor internal to the thermoelectric array, or a user selectable input, such as a switch or a thermostat, or any combination of these. In one embodiment, the controller operates in accordance with at least one algorithm responsive to the at least one input to control the at least one output.
Preferably, the at least one characteristic impacts the convective heat transport, and the adjustment improves efficiency or power output by adjusting the characteristic. For example, the control system varies movement of at least some of the convective medium in response to the input. In another embodiment, the control system adjusts other characteristics, such as the current through at least some of the thermoelectric elements. The adjustment of characteristics other than the convection may be alone or in combination with adjustment of the convection.
These and other aspects are described in more detail below in conjunction with the following figures.